1. Field of the Invention
The present invention relates to an optical focusing apparatus for the reading of a data medium moving past it. More particularly, it is applicable to the reading of data on magnetic discs of magnetooptical storages used mainly in information processing systems.
2. Description of the Prior Art
It is common practice for a user of data processing systems to either immediately use and analyze the result of operations performed by the central processing unit of these systems or to store it for varying periods of time in storages often called bulk storages.
Among bulk storages in current use, magnetic disc storages are frequently employed, including magnetooptical storages. In disc storages, the data is carried by magnetic discs and are stored within concentric recording tracks usually covering the major portion of their two faces.
In general, a sequence of magnetic data stored on the track of a disc offers itself in the form of a succession of small magnetic domains called "elementary domains" distributed throughout the length of the track and which have magnetic inductions of the same module and are of opposite direction.
For the sake of simplicity, the term "transducers" is used to identify the means which enable one to write (or store) the data on the discs or to read data from the disc or to eventually perform one or the other of these two functions. As a rule, one or more transducers are associated with one face of a given disc which moves past the latter.
In the development of magnetic disc storages, one of the current tendencies is to orient oneself toward the study of magnetooptical storages where the data is written on magnetic discs by magnetic or optical means, the reading being effected by a group of optoelectronic means which enable each one of them to observe at a given moment and at a given location one face of a disc by means of a beam of polarized light and to deliver an electrical signal whose voltage or current is dependent upon the value of the data stored at that location.
In these storages, the mode of reading data is based on the principle of interaction between a rectilinear polarized light with the magnetic state of the layer constituting the magnetic disc, said interaction resulting in the rotation of the electric field vector in the plane perpendicular to the direction of propagation.
It is recalled in this context that a light is polarized rectilinearly in the plane when the electric field vector (and, hence, the magnetic induction vector) always maintains the same direction in the plane normal to the direction of propagation of the radiation.
Optical devices for reading data stored on a magnetic medium and which use the interaction principle mentioned hereinabove are known. Such a device is especially described in U.S. application Ser. No. 417,904, filed Sept. 14, 1982, (now U.S. Pat. No. 4,510,544) and corresponding to French Application No. 81.19543 filed on Oct. 16, 1981 by Compagnie Internationale Pour L'Informatique Cii Honeywell Bull, assignee of the invention therein described under the title "Optoelectronic Device for Reading Data Stored on a Magnetic Disc".
More particularly, this optical reading system comprises:
a source of light for transmitting a beam of polarized light over the surface of the disc at a location determined by the latter and whose interaction with the magnetic state of the medium at that location (i.e., with its physical state defined by its magnetic properties) produces a rotation of the plane of polarization of the light,
an optical system for focusing the beam of light, having a large field of observation and affording the possibility of observing simultaneously a plurality of tracks and a plurality of data on each track,
analyzing means provided with photodetectors (e.g., photodiodes) comprising a light meter to enable the detection of the angle of rotation of said plane and which deliver an electrical signal whose voltage or current is dependent upon the value of the data stored at that location, and
means for projecting the image of the surface of the disc which is observed by the focusing lens onto a plane P on which the photodetectors are arranged.
In one preferred embodiment of the optical reading means whose main constituent elements have been noted above, most of these elements are arranged on a moving platform so as to permit the latter to move opposite the tracks of the disc in order to be able to observe all tracks. This platform comprises a supporting arm to which is attached by one of its ends, for example, a moving carriage which lies outside the disc and whose other end is integral with a suspension means carrying a main body, containing, for example, at least one transducer for writing data of the magnetic disc.
For a better understanding of the invention, it will be helpful to review several features of the platforms most frequently used. Generally, but not exclusively, the main body of a platform has the form of a relatively flat rectangular parallelepiped whose first "large face" opposite the disc face contains the writing transducer carried by this main body, the second "large face" substantially parallel to the first face containing the ends of the input and/or output conductors of said transducer.
During the rotation of the disc, a cushion of compressed air is formed between the latter and the first "large face" of the main body which prevents this body from touching and damaging it. The main body is then said to fly above the disc. The distance between the transducer and the face of the disc is called "flight altitude" or "flight height" of the main body. Ordinarily, it is of the order of several tenths of a micron and remains substantially constant during the entire flight of the main body above the disc.
The air cushion exerts a pressure on all or part of the first large face of the main body normal thereto and directed toward its second large face. "Flight surface" is the portion of the first large face subjected to the pressure of the air cushion. The resultant force, called "lift" of the main body, is dependent upon the flight surface. The dynamic equilibrium of the main body in flight is obtained by opposing the lift with an equal and opposite force called "loading force", usually applied to the second large face.
In current practice, platforms known under the trade name "Winchester" are rapidly gaining favor and are being widely applied. These platforms are manufactured by Applied Magnetic Corporation (A.M.C.) under reference numbers 3304 and 3306.
A Winchester-type platform is manufactured in the following manner: One or more grooves, also called slots, are provided on the lower face of its main body having a depth greater than or equal to 30 microns and which can reach up to several tenths of a millimeter. This results in the presence of on said face of several projecting parts called "slides" which usually have the form of skis.
The surface of the slides lying opposite the face of the disc constitutes the flight surface. Since the lift is proportional to said surface, it is obvious that the grooves made in the first large face of the main body, by reducing the flight surface (in relation to a main body which has no grooves), permit the reduction of the flight and, thereby, of the loading force. The latter is relatively small and is of the order of 10 to 20 grams.
Generally speaking, the flight of a main body of a Winchester-type platform above the face of the disc associated therewith is effected in such a way that the surface of the slides is slightly inclined in relation to the face of the disc, thus providing these slides with a lift which is sufficient to ensure a relatively stable flight of the main body.
The assembly made up of the optical system for focusing the beam of light, having a large field of observation, and the main body of the platform is called the optical focusing system. Thus, the latter is an integral part of the optical reading means of the magnetic disc.
One of the principal problems to be solved for any optical focusing system is to focus with a high degree of accuracy the beam of a polarized light transmitted by the light source in such a way as to obtain an extremely clear image of the disc surface. Put differently, this means that the distance between the disc and the optical focusing system must remain constant and substantially equal to the focal distance of said optical systems.
Servo focusing devices exist which enable the solution to the above-mentioned problem. Such a servo focusing means is, for example, described in a paper by Nobutake IMAMURA and Chui Chi OTA entitled "Experimental Study On Magnetooptical Disc Exerciser With Laser Diode And Amorphous Magnetic Films" published in the Japanese Journal of Applied Physics, Volume 29, No. 12, December 1980, pp. 731-734. The object of such a servo focusing device is that, whatever the oscillations of the disc surface may be (the disc surface is never flat and the disc always has a certain radial runout during rotation), the beam of polarized light is always focused accurately.
In such a device, the optical focusing system is mounted on fluid bearings with air and is integral with the moving electrodynamic coil of a linear electromotor. A portion of the reflected beam of light is transmitted by means of a cylindrical lens having a bridge of photodiodes, two of which are located along a vertical axis and the other two are placed along a horizontal axis. It is assumed that the sum of the two signals delivered by the two vertical photodiodes equals A and the sum of the signals delivered by the two horizontal photodiodes equals B. It is the difference between the signals A and B which controls the current flowing through the moving coil of the linear motor that enables the optical focusing system to move. If the beam of light is perfectly focused onto the disc surface, the difference between A and B is zero. Thus, it is inadvisable to move the optical focusing system and no current is sent through the moving coil of the linear electrodynamic motor. On the contrary, when the optical focusing means is too close to the disc surface, the signal A is higher than the signal B and a current is sent through the coil of the motor so that the motor is displaced as the optical focusing system is moved from the disc. On the other hand, when the optical focusing system is too far from the disc, the signal B is higher than the signal A and a current is sent through the coil of the electrodynamic motor so that the optical focusing system comes closer to the disc.
Such servo focusing devices have the disadvantage of being relatively bulky and are thus difficult to mount on a platform such as the one described above. On the other hand, they must be extremely precise and are thus burdensome.